1. Field
The subject invention relates to plasma processing chambers and, in particular, to a coating for various parts of a plasma processing chamber, which enhances the corrosion resistance of the parts in the presence of active plasma species.
2. Related Art
In plasma processing chambers, a showerhead is often used to inject the process gas. In certain plasma chambers, such as capacitively-coupled plasma chambers, the showerhead may also function as an electrode, coupled to either ground or RF potential. However, during processing the showerhead is exposed to the plasma and is attacked by the active species within the plasma, such as halogen plasma of CF4, Cl2, etc. This phenomenon is especially troublesome for showerheads having a chemical vapor deposited silicon carbide coating (CVD SiC).
Another chamber part that is exposed to plasma is the chuck, such as an electrostatic chuck (ESC). The ESC generally functions as the lower electrode for the RF power and the wafer holder. Being exposed to the plasma within the chamber, the ESC suffers plasma corrosive attack and wafer abrasion in the plasma processes. Therefore, the surface of ESC has to be hard and stable in the plasma etching process. However, the current ESC surface is usually made of solid ceramics, such as Al2O3 or AlN pucks, which can be eroded by plasma and induce contamination during plasma processes.
Other parts contacting plasma within the chamber may be, for example, focus ring, plasma confinement ring, chamber liner, etc. Of particular interest is the parts made of anodized aluminum.
Various coatings have been proposed and tested in the prior art for protecting the showerhead and ESC from plasma erosion. Yttria (Y2O3) coating is believed to be promising; however, it has been very difficult to find a process that results in good coating, especially one that does not crack or generate particles. For example, there have been proposals to use plasma spray (PS) to coat chamber parts made of metal, alloy or ceramic. However, conventional PS Y2O3 coating is formed by the sprayed Y2O3 particles, and generally results in a coating having high surface roughness (Ra of 4 micron or more) and relatively high porosity (volume fraction is above 3%). The high surface roughness and porous structure makes the coating susceptible to generation of particles, which may contaminate the wafer being processed. In addition, particles will come out from the gas holes and drop on the wafer when the as-coated shower head is used in the plasma process, as the plasma sprayed coating inside the gas hole is very rough and poorly adheres to the substrate.
In addition, PS Y2O3 is usually deposited on Al alloy parts whose surface has been previously anodized. Since the adhesion of PS Y2O3 to anodized surface is poor, the anodized layer has to be removed from the parts prior to PS Y2O3 deposition, which increases the production cost. That is, in the prior art it is conventional to anodize and then seal the anodized chamber part. Then, the anodization is removed from the area of the part that would be exposed to plasma inside the chamber. The now exposed area is coated with PS Y2O3, so as to avoid adhesion problems of yttria to anodized aluminum.
Another shortcoming is the poor structure stability of thick PS Y2O3 coating that tends to crack when the service temperature is increased, as the thermal expansion coefficient of Y2O3 and Aluminum alloy are quite different.
Other proposals for forming Yttria coating involve using chemical vapor deposition (CVD), physical vapor deposition (PVD), ion assisted deposition (IAD), active reactive evaporation (ARE), ionized metal plasma (IMP), sputtering deposition and plasma immersion ion process (PIIP). However, all these deposition processes have some technical limitations such that they have not been actually used to scale up for the deposition of thick coating on the chamber parts for the plasma attack protections. For instance, CVD of Y2O3 can not be carried out on substrates that cannot sustain temperatures above 600° C., which excludes the deposition of plasma resistant coating on chamber parts that are made of aluminum alloys. PVD process, such as evaporation, can not deposit dense and thick ceramic coating because of their poor adhesion to substrate. Other deposition processes can not deposit thick coating either due to the high stress and poor adhesion (such as sputtering deposition, ARE and IAD) or the very low deposition rate (such as sputtering deposition, IMP and PIIP). Therefore, so far no satisfactory coating has been produced, that would have good erosion resistance, while generating low or no particles and can be made thick without cracking or delamination.
In view of the above-described problems in the art, a solution is needed for a coating that resists plasma species attack and does not generate particle or cracks. The coating should have acceptable roughness and porosity values, so that it could provide long service life. The process for fabricating the coating should allow thick coating without being susceptible to cracking or delamination.